THE MIRACLE OF YEAST

"Nothing about wine is more lasting -- or astonishing -- than fermentation." Matt Kramer, Making Sense of Wine

The process of making wine is simple. Single cell plants of the genus Saccharomyces consume sugar in grape or other fruit juice and transform it into approximately equal parts of alcohol and carbon dioxide. It is the single celled plants that we commonly call yeasts that are the real winemakers. The humans who usurp the name winemaker are largely technicians.

Roger Boulton, University of California (at Davis) Department of Viticulture and Enology, stated the facts succinctly. "Ninety percent of winemaking has nothing to do with the winemaker. All a winemaker is doing is preventing spoilage, introducing some style characteristics and bottling it." In other words, the yeast is making the wine.

To quote Matt Kramer again, "...to really understand how wine gets made, as opposed to preserved, you need only understand fermentation. Everything else is flourish." This, then is the story of yeast and how these single-celled plants -- a fungi, really -- transubstantiate source material, whether it be grape juice or oak leaves, into wine.

If there is an art to winemaking, and there certainly is, then it is the art of controlling yeast. It is the art of selecting the appropriate yeast, introducing it at the correct moment, feeding and nurturing it so as to coax it into living, reproducing and dying in a prescribed manner, and then cleaning up after it so as to preserve the fruit of its labor. It is the art of controlling its temperature, the amount and kind of air it is allowed to breathe, and feeding it the sugar and other nutrients it needs to serve man. For it is not in the nature of yeast to serve man, but rather yeast exists to serve yeast. Controlling yeast is the real art of making wine.

Unravelling the Great Mystery

"The phenomenon of fermentation must surely have beenone of the longest-standing mysteries of civilization."Matt Kramer

We really have no idea where the first serendipitous batch of wine was made or what the person who discovered it thought of the resultant liquid, but the result must have been favorable because men worked very hard to replicate what undoubtedly first occurred as an accident. Certainly early man discovered there were three ways to consume grapes -- raw, dried (raisins) or as juice. Raw grapes did not last too long, and grape juice lasted only slightly longer. We can only imagine that someone left a container -- a skin or bladder bag, clay jar, or closely woven and treated basket -- of grapes or grape juice sitting out and wild yeasts began to ferment it. One can imagine that the smell of fermenting grape juice repelled the owner, but in due course the smell became more agreeable. In time, the liquid was consumed, probably in small, experimental amounts, and no deleterious effects noted. It was probably noted that the resulting liquid would keep much longer than juice that had not been allowed to "spoil," and fermentation was then sought as a preserving measure for the juice. The effects of the alcohol were probably discovered when a very thirsty soul drank too much of the fermented juice and got drunk, but that is another matter entirely. The early and long-term importance of fermented grape juice would have been that of it's preserving effects.

That yeast was involved was not known until several thousand years later. In 1680 Anton van Leeuwenhoek, Dutch naturalist and inventor of the microscope, turned his attention to yeast cells and discovered that some kind of microscopic life existed. His microscope was not powerful enough, however, to allow him to characterize the life or connect it to fermentation. In 1785 the French chemist Antoine Laurent Lavoisier postulated that alcohol fermentation was a chemical process. Sugar had already been identified as the raw material of fermentation. Lavoisier noted that between the alcohol and carbon dioxide gas produced during fermentation, all fermentable sugar was accounted for. Thus, he concluded that fermentation was caused by the sugar molecule being chemically split, an imaginative and reasonable assumption for that particular period. It was not until 1835 that Charles Cagnaird de la Tour (France) and Schwann (Germany) independently turned their much improved microscopes to the deposits left in beer vats. There they noted single-celled creatures multiplying before their very eyes by budding (by james key). Charles Cagnaird de la Tour concluded that fermentation was the result of yeast growth rather than merely a chemical process as formerly believed. He was, it turns out, only partially right.

In 1839, Justus Freiherr von Liebig, a German chemist who made significant contributions in understanding organic compounds, turned his microscope to some fermenting beer. He wrote, "Beer yeast, when dispersed in water, breaks into an infinite number of spheres. When these spheres are transferred to an aqueous solution of sugar, they develop into small animals. These are endowed with a sort of suction trunk with which they gulp up sugar. Digestion is immediate and clearly recognizable because of the discharge of excrements.

"These animals evacuate ethyl alcohol from their bowels and carbon dioxide from their urinary organs. Thus, one can observe how a specially lighter fluid is exuded from the anus and rises vertically whereas a stream of carbon dioxide is ejected at very short intervals from enormously long genitales." This humorous description, however anatomically and technically flawed, is still too priceless to be forgotten. Whenever you wonder what your yeast are doing, think of von Liebig's description and smile.

In 1846 the Swedish chemist Jons Berzelius coined the word "catalyst" to describe a process by which one substance accelerates a change in another substance without being changed in the process. Berzelius reasoned that yeasts were actually catalysts whose mere presence in a sugary liquid caused it to ferment. In other words, no biological activites were involved. Justus von Leibig, a German chemist, supported Berzelius' theory, but with a refinement. He held that it was the process of decay of the dead yeast cells which acted as the catalyst for fermentation and caused sugar to be converted into alcohol and carbon dioxide. In combating supporters of the biological school of fermentation, von Leibig pointed to soured milk as proof of a chemical cause, noting the absence of yeast in souring milk. In 1857, none other than the 34-year old Louis Pasteur entered the fray. Through careful microscopic studies, he had found tiny cells reproducing in soured milk which he thought to be lactic acid yeasts. They are, in fact, bacteria, but the important thing is that they were alive. Two years later, in 1857, Pasteur presented his Note on Alcohol Fermentation, where he showed that the growth and reproduction of yeasts was the cause of fermentation. The theory of "spontaneous generation" of alcohol and carbon dioxide through the catalytic presence of yeast was laid to rest. We now know that Pasteur was partially correct and partially wrong, but that's getting ahead of the story.

Pasteur went on to discover something really amazing about yeast cells -- they can live without oxygen. From this he concluded, "Fermentation is life without oxygen." This is not true, of course, because we now know that yeasts cause better fermentation in the presence of oxygen than without it, but Pasteur at least recognized the extreme possibility when no one else did. Pasteur died before two German brothers, Eduard and Hans Buchner, demonstrated in 1897 that yeasts, per se, do not cause fermentation. While searching for an extract for a medicine, the Buchner brothers ground up yeast cells with sand and silica and squeezed out the "extract" in a hydraulic press. Needing a way to preserve the "extract," they recalled how well high-sugar content solutions such as jam and syrup were preserved and decided to use pure sugar. However, when the "extract" was applied to sugar, the sugar fermented. Eduard Buchner would receive the Nobel Prize for his discovery of "cell-less fermentation," although another German had already postulated and named the process.

Wilhelm Kuhne, in 1878, recognized that a catalyst was indeed at work in yeast to cause fermentation. Although unable to isolate the catalyst as the Buchners later did, he had at least named it. The catalyst, he said, was contained in the yeast, and thus he coined the word enzyme to describe it from the Greek en (in) and zume (yeast). He was, indeed, entirely correct. It is the enzymes secreted by yeast cells that act upon sugar molecules and create the process known as fermentation. The enzymes, plural because we now know that over two dozen enzymes are involved in creating some 30 chemical reactions, are the catalysts that transform sugar molecules into alcohol and carbon dioxide gas. These reactions are ordered, one dependent upon the preceeding one to succeed. Protease works only on proteins; invertase breaks down sucrose; and so on until zymase creates alcohol itself. And so the process is chemical after all, but it is also vitalistic since yeast is required to manufacture the enzymes.

As it was in the beginning, winemaking is still dependent upon this tiny organism. A mere 8/25,000ths of an inch long, the yeast cell is an unlikely factory. And it wouldn't be a factory at all if it didn't reproduce at such a fantastic rate. One drop of fermenting juice can contain five million yeast cells capable of doubling their number every two hours in absolutely perfect conditions. In a large vat, the amount of carbon dioxide given off can cause the surface of the juice to froth and roil with a buzzing hiss that can be disquieting.

A Very Local Affair

Wild yeasts are to wine what weeds are to a garden.

There are thousands of yeasts blowing in the wind and living in the soil and on leaves, stems, vines and grapes. Of the 15 known genera, one in particular is favored for winemaking. From the Greek words for sugar (sakchar) and fungus (Mykes), Saccharomyces is the genus that makes up most yeasts used in making bread, beer and wine. Seven species make up the genus, and the most important of these is Saccharomyces cervisiae, also called S. ellipsoideus, and its varieties. Of the many non-Saccharomyces yeasts, all but one are decidedly bad for wine. At the moment, however, we are only concerned with the good yeasts.

For centuries, winemakers have been returning the pressed-out skins, seeds and pulp (pomace) of the wine grapes to their vineyards as fertilizer. The pomace is rich with yeast, and that used in making red wines is extremely rich in whatever strain dominated the primary fermentation. By returning the yeast-rich pomace to the vineyard, the winemaker alters the natural mix of yeasts locally. Over time, the selected strain dominates so firmly that the winemaker can achieve precisely predictable results by simply encouraging spontaneous fermentation of newly pressed must. For the winemaker in Ohio or Texas or Washington planting new vines where none have grown before, this is an envious situation. Even in the established vineyards of New York and California, such domination by a primary Saccharomyces is perhaps centuries away.

To understand how truely amazing this is, and to better our winemaking knowledge, it is necessary to look at the natural mixture of yeasts that are found on grapes (and plums, cherries, peaches, apples, etc.) almost anywhere on Earth.

The Good, the Bad and the Ugly

The secret of good winemaking is good ingredientsand absolute control over all conditions

If one takes a pint of grapes from any vine almost anywhere in the world and crushes them, 50-75% of the yeasts naturally present will be one or more strains of Saccharomyces cerevisiae, Hansenula, Cryptococcus, Rhodotorula, Kluyveromyces, Pichia membranefaciens, and Torulaspora delbrueckii. One will also probably find at least 100 colony forming units per milliliter of Kloeckera apiculata (Hanseniaspora), Metschnikowia, Candida stellata, and Candida pulcherrima. Finally, a few Brettanomyces, Dekkera and Zygosaccharomyces bailii may also be present in the must.

When yeasts enter a new environment, they exist for a while in what is known as a lag period. During this period, which can last from minutes to hours, the yeasts basically test the environment to determine whether or not it is suitable for colonization. If they determine that it is, they end the lag period and begin reproducing. Yeast can reproduce at an alarming rate in a favorable environment. This rate, at least for a period, is typically one logarithmic unit per two hours, meaning that 1 yeast cell can theoretically become 8 in six hours, 64 in 12 hours, 512 in 18 hours, and 4096 in 24 hours. Luckily, when their population density reaches about 150,000,000 per milliliter of host liquid, they settle down and maintain a relatively steady population. This continues, all other conditions remaining constant, until they deplete all available oxygen, use up all available nutrients, or the alcohol they produce becomes intolerably concentrated. Most then die off, but if both oxygen and nutrients are still available then Brettanomyces, Dekkera and Zygosaccharomyces bailii, if present, may now decide to end their lag period and begin multiplying, ruining what would otherwise be an almost finished wine.

There are several methods to ensure that only the preferred Saccharomyces yeasts colonize the must. If spontaneous fermentation (using indigenous yeasts) is being encouraged, raising the temperature of the must to 70-75 degrees Fahrenheit for the initial, aerobic period in the primary will overwhelmingly favor Saccharomyces yeast. The temerature should be reduced during the anaerobic period of alcohol fermentation, but maintained above 57.2 degrees Fahrenheit to discourage Kloeckera (Hanseniaspora) and others from gaining an advantage. Once the wine has been fermented to dryness and bottled, it can tolerate much lower temperatures without risking non-Saccharomyces yeast growth. If bottled sweet without stablization, however, the risk is still present.

Non-Saccharomyces yeasts are less tolerant of SO2 (sulfite) than strains of Saccharomyces cerevisiae. The addition of 30-50 parts per million (ppm) of SO2 to the freshly pressed must or crushed grapes limits the growth of non-Saccharomyces yeast. The Saccharomyces yeast gains an advantage and grows from less than 1000 cells per mL to between 1,000,000 and 2,000,000 cells per mL within 24 hours, thereby dominating the fermentation. SO2 treatment, combined with a 70-75 degrees F. temperature, ensures the domination of the desired yeast during the primary (aerobic) phase. By cutting off the oxygen supply with an airlock and lowering the temperature to 60-65 degrees F., a controlled, anaerobic, secondary phase in which alcoholic fermentation is optimized is obtained. Stabilizing the wine before bottling prevents further fermentation.

The surest method of controlling yeast variety is to pasteurize the must to 150 degrees F. to kill all indigenous yeast and bacteria, cool it to around 68 degrees F., and then inoculate it with the desired yeast. This is not as romantic as promoting spontaneous fermentation of the yeasts the grapes themselves bring to the press, but for the average person it sure is less risky. But pasteuriztion ruins the must of many potential winemaking ingredients and should not be practiced unless specified--or unless you are willing to risk the batch by experimenting.

The next best method of controlling yeast variety is to add an appropriate dose of potassium metabisulfite, either as one crushed Campden tablet to each gallon of must or as 1/4 teaspoon of the "K meta" salt itself to each 5 gallons of must, 24 hours before inoculating with the desired strain. That is the method generally followed in the recipes posted on this website, except when sufficient heat is used, either by cooking the fruit or boiling added water, to extract juice and flavor and set the desired color. A small percentage of the populace have genuine but often misguided aversions to adding chemicals of any sort to their wine. The only safe alternative to treating the must with SO2 is pasteurization which, as we have seen, is often unacceptable for winemaking purposes.

Cultured Wine Yeasts

In the New World, the rule is to use cultured yeasts

By isolating and culturing the dominate yeast strains from a variety of favorable locales, scientists have made it possible for any home winemaker to select the yeast with which he or she inoculates the must. One can choose a yeast tolerate of high or low temperatures, that produces abundant or little foam, that works very fast or very slow, that changes or preserves natural flavors and characteristics, that impart certain desirable or no undesirable off-flavors, that lay down compact or loose lees, or that work particularly well in producing particular types of wine. This is a decided boon for the home winemaker, and those who do not recognize and exploit it are sadly limiting their experience and potential. Given the right grape, yeast and instruction, anyone can make almost any type of wine right in their own home. Lastly, non-grape wines can be made with as much exactitude and style as grape wines by using specific yeasts best suited to the specified ingredients.

Most commercially cultured yeasts come in small or large quantities. Cultures are available in several forms, but the most convenient form for the average home winemaker is in the Active Dry Yeast (ADY) form. The yeast is in a dehydrated, dormant state yet very much alive. The most convenient quantity is the 5-gram foil packet, or sachet, suitable for quickly inoculating one to five gallons of must or for making an activated starter. Of course, these same packets can be used to inoculate larger batches, but it may take several extra days for the culture to multiply sufficiently. In theory, one yeast cell is all that's required to inoculate a 10,000-gallon vat, but it takes too long to breed the required density and the must could spoil long before such a density is achieved.

Active dry yeast (ADY) cultures are not freeze-dried cultures as I once thought. Malo-lactic bacteria cultures are freeze-dried under vacuum at sub-zero temperature, but yeast is fluid-bed dried. This type of drying is accomplished by extruding 70% moisture compressed yeast through a perforated plate into a spaghetti-like form, about the diameter of a 0.036 inch pencil lead, into a drier with a screen bottom that has a upward flow of air that keeps the particles of yeast suspended in a fluid-like bed. The incoming air is controlled for volume, temperature and relative humidity. The drying from the original 70% moisture down to 4-7% occurs in less than 30 minutes. The average temperature of the yeast particles in the fluid-bed is 86 degrees F. throughout the drying period. The dried yeast is immediately put under nitrogen and refrigerated until time of packaging.

Yeast Starters

An active culture gets the fermentation up to speed in hours

The preferred method of adding an ADY culture to a must is to add a yeast starter, or activated culture to the aseptic must. This simply means the yeast is introduced to a liquid medium favorable to rapid activation and propagation a day or so prior to adding to the must. The liquid with the activated culture is then added to the must as required, where the yeast culture very rapidly propagates to a desired density.

This method is preferable to adding the ADY culture to the must for several reasons. First and foremost, it results in a rapid fermentation. The flavors, aromas and nuances we want to capture from the must and impart into our wine are often very perishable and dissipate or change within days if not hours. The sooner the yeast can get to work capturing them, the better the resulting wine will be as a result. Adding a starter, as opposed to adding the ADY culture directly from the foil packet, can save one to several days, depending on the yeast strain and the size of the batch of must.

Secondly, it ensures viability of the strain. Normally, when you purchase a sachet of yeast you have no idea how old the ADY culture inside the packet is. Given a constant and acceptable temperature, the culture can survive for years in the foil without detriment. But the foil packets could have been--and probably were--shipped without regard to temperature. The box in which they were shipped could have sat in the sun on the tarmac at Los Angeles International in 110 degrees heat for an hour before being loaded in the plane that took it to your regional airport hub or local point of entry. It was then taken by truck to a transshipment warehouse where it may have dwelled for days in similar heat before being trucked to your city and then to your supplier. If 90% of the culture baked in the process, it will take that much longer for the culture to build to a density conducive to your needs. If 100% of the culture baked, you could easily waste a week discovering that fact, and during that week your must deteriorates and possibly is ruined. By making a stater solution two days before needed, you would have discovered that the yeast was non-viable within a day and still had time to prepare another.

Thirdly, a starter properly made, using water, a small quantity of the must itself or a juice substitute (grape, orange or apple juice) and some nutrients, will acclimate the yeast to its destined environment. When the starter is added to the primary, it will practically explode with activity and do what nature and selection programmed it to do and do it that much more efficiently.

The correct method of making a starter is to rehydrate the yeast, activate its life cycle, and add it to the must. The optimum way to rehydrate the yeast is to add it directly to 1 cup of 100-105-degree F. tap or spring water (the harder the water the better; do not use distilled water). Stir gently, cover, allow to rehydrate for at least 30 minutes, check on it to be sure it is viable, and then leave it another 3 1/2 hours. During this time, allow the starter and must (or fruit juice) to attemperate to within 10 degrees F. of one another, and then add to the starter 1/4 cup of pre-sweetened, reconstituted juice (not pure concentrate) or strained must. Re-cover the starter, set it in a warm place and leave it alone. Check on it 4 hours later to ensure it is viable and add to it another 1/4 cup of juice or strained must. Again, cover and leave it alone for 4 hours. You can now add it to the must or add another 1/2 cup of juice or strained must to really increase the yeast population (at the end of an additional 4 hours, the colony will be approximately 64 times as large as it was when rehydrated). For highly acidic (native grapes) or potentially troublesome musts or juices (like blueberry, peach, or Ribena blackcurrant), the more must you add to the starter, the better acclimated the yeast will be to the conditions they will be living in. There are other methods of starting a culture and most are just as successful, but this method, only slightly varied, was recommended by George Clayton Cone of Lallemand, the makers of Lalvin wine yeasts, and that is good enough an endorsement for me.

Lallemand's scientists found that some musts and juices contain sprays, toxins and excessive SO2 that can be detrimental to the activity of yeast. The dry yeast is like a sponge for the first few seconds in liquid and will absorb everything into the cell that it would normally reject in the rehydrated form. Many home winemakers add the ADY culture directly to the must or juice and get away with it. However, many times it is the beginning of a sluggish or stuck fermentation. There are over 150 billion yeast cells in a 5-gram packet of Lallemand yeast. If you kill off half of them by improper rehydration, you still have 75 billion cells to work with. This 75 billion will go on to do a good job most of the time, but whatever killed off the other 75 billion may have seriously affected the health of the survivors. Can you spell "stuck fermentation?" A little prudence is good insurance.

If you forget to make a starter or simply don't want to, then inoculate the must by sprinkling the ADY culture evenly over the top of the must and DON'T stir it in. Cover the primary and take a peek 12 hours later. If viable, there will be a prominent yeast colony across the surface and evidence in the form of a thin foam and/or a distinctly yeasty smell. Stir it shallowly into the must and 12 hours later stir it deeply. If there is no evidence of the yeast's viability, wait another 12 hours and check again. If still no evidence, inoculate again. Better yet, make a starter. Better late than sorry.